In industrial vacuum installations, use is made of a plurality of pumps coupled in series, comprising a primary pump and one or more secondary pumps. These secondary pumps are generally rotary pumps, e.g. pumps of the "Roots," "screw", or "spiral" types. Pump power is limited by thermal and mechanical constraints, and since said power is a function of the pressure difference between the pump's inlet and outlet, these secondary pumps cannot be started at atmospheric pressure. It is necessary to begin by starting the primary pump in order to bring the pressure down to a value which is acceptable for the secondary pump.
In order to determine the instant at which the pressure reaches said acceptable value, a pressure gauge is connected in conventional manner on the suction duct, said gauge being constituted by a pressure-sensitive capsule which, on being deformed, causes an electrical contact to be closed. This contact is connected in the control circuit of the secondary pump and serves to cause it to be started.
This type of pressure gauge withstands corrosion and dust-laden gases poorly, and at low pressures it is sensitive to vibration. Consequently, reliability is poor and this may mean that the secondary pump does not start, or worse it may cause it to start at too a high pressure with the consequent risk of the pump seizing.
The present invention consists in removing the pressure gauge, and thus in not measuring pressure directly, but instead, measuring a physical magnitude which follows a law similar to that of pressure over a given range.